Everything contained in terrible poem (6)
I believe the answer is:
ballad
'poem' is the definition.
(I know that ballad is a type of poem)
'everything contained in terrible' is the wordplay.
'everything' becomes 'all' (all things).
'contained in' indicates putting letters inside.
'terrible' becomes 'bad' (similar in meaning).
'all' going within 'bad' is 'BALLAD'.
(Other definitions for ballad that I've seen before include "Narrative song with a recurrent refrain" , "Poem telling a story" , "Narrative song of popular origin" , "Form of verse" , "Narrative poem or song" .)